Denso: Understand Your Spark Plugs To Understand Your Engine's Performance

Denso Spark Plugs - Diagnosis of a Used Spark Plug

If you're seeing any of the following, you probably don't have the manufacturer's recommended spark plug installed. But the symptoms are separate from the causes and all the components in an ignition system, no matter how small, will leave some signs on the spark plugs over time if they're not in harmony. This guide can provide insight to the engine's performance.

	Normal Light grey or tan deposits and slight electrode erosion
	Carbon Fouling Dry, soft black carbon on the insulator and electrodes. Symptoms: Poor starting Misfiring Faulty acceleration Causes: Faulty choke - over rich air/fuel mixture Delayed ignition timing Bad ignition leads Plug heat range too cold
	Pre-Ignition A melted or burned center and/or ground electrode, blistered insulator and aluminum or other metallic deposits on the insulator Symptoms: Loss of power causing engine damage Pre-ignition occurs when combustion begins before the timed spark occurs. Causes: Plug insufficiently tightened Engine insufficiently cooled Ignition timing too advanced Plug heat range too hot
	Over Heating An extremely white insulator with small black deposits and premature electrode erosion. Symptoms: Loss of power at high-speed or during heavy load Causes: Plug insufficiently tightened Engine insufficiently cooled Ignition timing too advanced Plug heat range too hot
	Mechanical Damage Bent electrode and a broken insulator, dents often present on electrode Symptoms: Misfiring Causes: Plug nose is too long for engine head Foreign object (bolt/nut) in combustion chamber
	Oil Fouling Wet, oily black deposits on the insulator and electrodes Symptoms: Poor starting Misfiring Causes: Wrong piston rings, cylinders, and valve guides Fuel mixture oil content too high (two-stroke engines)
	Broken Insulator Insulator is cracked or split Symptoms: Misfiring Causes: Severe detonation Incorrect tool/torque applied during installation or removal Careless gap setting
	Torched Seat Melted in the thread and seat area of the plug housing Symptoms: Loss of power causing engine damage Causes: Plug insufficiently tightened